You can't directly measure the size of a rock using a measuring cylinder. Measuring cylinders are designed to measure the volume of liquids, not the dimensions of solid objects.

However, you can use a measuring cylinder to indirectly determine the volume of an irregularly shaped rock by using the principle of displacement:

1. Fill the measuring cylinder partially with water: Make sure the water level is easily readable.

2. Note the initial water level: Record the volume of water in the cylinder.

3. Carefully submerge the rock: Place the rock into the measuring cylinder, ensuring it is completely submerged.

4. Observe the new water level: The water level will rise as the rock displaces water.

5. Calculate the volume of the rock: Subtract the initial water level from the final water level. The difference represents the volume of the rock.

Important Note: This method only gives you the volume of the rock, not its length, width, or height.